fix: handle engagement: null in watchlist and YouTube normalizer

When X posts return engagement: null, dict.get("engagement", {})
returns None (key exists with null value), causing AttributeError.
Use `or {}` idiom to coalesce None to empty dict.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

scripts/lib/normalize.py

@@ -174,7 +174,7 @@ def normalize_youtube_items(
 
     for item in items:
         # Parse engagement
-        eng_raw = item.get("engagement", {})
+        eng_raw = item.get("engagement") or {}
         engagement = schema.Engagement(
             views=eng_raw.get("views"),
             likes=eng_raw.get("likes"),